A surface-coated cutting tool used for cutting steel, cast iron and the like generally includes a base material made of a tungsten-carbide-based cemented carbide and a coating covering the surface of the base material. The coating is a stack of two or more layers such as Ti compound layer and alumina layer. Here, the alumina layer which forms the coating has advantages of excellent oxidation resistance and heat-resistant stability as well as high hardness. Meanwhile, the alumina layer has a disadvantage that it has a relatively lower strength and is brittle than the Ti compound layer. Due to this disadvantage, chipping may occur to the cutting edge or wear of the cutting edge may increase, for example, when the cutting tool cuts a steel or cast iron under severe conditions such as high-speed cutting or high-speed and high-feed-rate cutting.
Trying to overcome this disadvantage of the alumina layer, Japanese Patent Laying-Open No. 11-138308 (PTL 1) for example provides different crystal structures formed respectively in the upper portion and the lower portion of the alumina layer. Specifically, the lower portion of the alumina layer is formed of a longitudinal diversified crystal structure and the upper portion of the alumina layer is formed of a longitudinal uniform crystal structure.
Further, according to Japanese Patent Laying-Open No. 2002-120105 (PTL 2), when an alumina layer is to be formed, a gas to which H2S gas and SO2 gas are added is introduced and further an increased amount of CO2 is introduced to form the alumina layer. Accordingly, the alumina layer is formed that has a crystal structure mainly constituted of α-crystal and satisfies the following relations. Namely, the ratio between x-ray diffraction peak intensity I(030) of the (030) plane that is a main peak and x-ray diffraction peak intensity I(104) of the (104) plane satisfies I(030)/I(104)>1, and x-ray diffraction peak intensity I(012) of the (012) plane satisfies I(012)/I(030)>1.
Here, the α-alumina with its orientation in the (030) plane has a higher crystallographic density than the α-alumina with its orientation in the (104) plane. Therefore, the x-ray diffraction peak intensity of the (104) plane can be increased to thereby form the α-alumina crystal formed of high-density crystal, as described above.
Japanese Patent Laying-Open No. 07-216549 (PTL 3) discloses an alumina layer having a single-phase α-structure textured in the (110) direction of x-ray diffraction so that texture coefficient TC (hkl) has a value larger than 1.5. This alumina layer has good adherence to the underlying base material and therefore has an advantage that the wear resistance is excellent.
European Patent Publication No. 1655387A1 (PTL 4) discloses an alumina layer having a texture coefficient TC (110) of the (110) plane of more than 2 and a texture coefficient of a crystal plane other than the (110) plane of less than 1.5. Further, according to PTL 4, an alumina contact layer which is a lower layer of the alumina layer also contains Al so that the bonding strength between the alumina contact layer and the alumina layer is increased.
Japanese National Patent Publication No. 09-507528 (PTL 5) discloses an alumina layer having a thickness of 2.5 to 25 μm and a crystal grain size of 0.5 to 4 μm. This alumina layer has a texture coefficient TC of larger than 2.5 and has a single-phase α-structure textured in the (104) direction. The alumina layer having such a crystal structure exhibits a property that it is excellent in wear resistance and toughness.
Japanese Patent Laying-Open No. 10-156606 (PTL 6) discloses a surface-coated cutting tool including a base material with its surface coated with an inner layer which is further covered with an alumina layer. According to PTL 6, in addition to a non-oxidizing gas component which is a main component, an oxidizing gas is further introduced to form the inner layer. Accordingly, the (110) plane of the alumina layer exhibits a maximum x-ray diffraction peak intensity, and lattice stripes of the alumina layer and the inner layer continue at the interface therebetween.
As an approach for improving the strength of the alumina layer other than those explained above in connection with PTLs 1 to 6, there is also a technique of adjusting the thickness and the surface roughness of the alumina layer as well as the average grain size of grains constituting the alumina layer. For example, according to Japanese Patent Laying-Open No. 62-228305 (PTL 7), the alumina layer has a thickness of 0.5 to 5 μm and a surface roughness of not more than 1 μm so that the strength and the adherence of the alumina layer are increased.
Further, WO1995/019457 (PTL 8) discloses an alumina layer having a thickness of 2.5 to 25 μm and a grain size of its constituent grains of 0.5 to 4 μm. This alumina layer has a single-phase α-structure textured in the direction of the (104) plane. Japanese Patent Laying-Open No. 2002-205205 (PTL 9) also discloses an alumina layer with its thickness adjusted to 2.5 μm or less by using alumina grains having an average grain size of 2 μm or less. An alumina layer having such a thickness and such a grain size can be formed to thereby enhance the toughness of the surface-coated cutting tool.